9mm runs from .354 to .356 depending on the cut of the barrel on the particular weapon. .38 caliber is not a true .38 in., rather they run .355 in some Colts revolvers, .357 in Smith and Wesson revolvers to as much as .358 in the old .38 S&W. Quality control was lacking in the older handguns and kyou almost had to slug the barrel to know lfor sure what size projectile would work best. I have shot muzzle loading rifles for about 30 years but never the smaller calibers. Mine have een in .45, .50 or .58. I have seen and handled some of the little squirrel guns but have not seen one that was designed to shoot the miniball type slug. All of the ones I have seen were rifled for round balls and patches and had a real slow twist. They probably would not stabilize a miniball like you would expect. I'm not argueing with anyone I want you to know. I am just comparing notes. The little minnieball in question has really confounded me. Monty